In a horizontal electroplating line, generally, plural pairs of backup rolls and current-carrying rolls are provided, and horizontal electrodes are disposed above and below a steel strip between the respective two current-carrying rolls to electroplate the steel strip while supplying electricity. In general, each of the current-carrying rolls comprises an iron roll, and each of the backup rolls comprises a rubber roll. In this case, the surfaces of the rubber rolls are concavely deformed due to a difference in hardness between iron and rubber, thereby bending the steel strip along deformation. For example, when an iron roll is provided on the upper side, and a rubber roll is provided on the lower side, the steel strip is bent concavely upward, and pulled by tensile force to produce warpage of the steel strip in the width direction thereof. Warpage of the steel strip causes problems of nonuniformity in plating, etc.
As techniques for correcting warpage of a steel strip in the horizontal electroplating line, Japanese Patent Unexamined Publication No. 3-126890 discloses a technique in which widthwise warpage deformation of a steel strip is constantly detected in a plating bath, and backup rolls are shifted in the pass direction of the steel strip while maintaining the predetermined pressure on current-carrying rolls on the basis of the detection signal, to correct widthwise warpage of the steel strip.
The method comprising shifting the backup rolls according to the warpage amount of the steel strip has a problem in that it is very difficult to shift the backup rolls in the direction of movement of the steel strip while maintaining the predetermined pressure on the current-carrying rolls. In addition, when the backup rolls are shifted under supply of electricity, a spark occurred on a gap which is generated between the current-carrying rolls and the steel strip, thereby causing the problem of damaging the surfaces of the steel strip and the current-carrying rolls. This technique also has a problem in that when the steel strip significantly warps, the shift amount of the backup rolls is increased, and thus the pass line of the steel strip significantly changes, thereby causing a difference between the amounts of the deposits on the face and back of the steel strip.
An object of the present invention is to provide a method of correcting warpage of a steel strip in an electroplating line in which the above-described drawbacks are improved.